Draft: January 18, 2002
Question Three: What are the chemical mechanisms that create secondary organic
aerosols, what are their precursors, what are the environmental conditions needed to
create and sustain particles, and what are the organic substances in the particles?
1. Precursors
The ability of a given volatile organic compound (VOC) to produce SOA during its
atmospheric oxidation depends on three factors:

The volatility of its products

Its emission rate (atmospheric abundance)

Its chemical reactivity
Most VOCs do not form aerosol under atmospheric conditions due to the high vapor
pressure of their products. As a result of the above constraints, VOCs that for all practical
purposes do not produce organic aerosol in the atmosphere include all alkanes with up to
six carbon atoms (from methane to hexane isomers), all alkenes with up to six carbon
atoms (from ethane to hexene isomers), benzene and many low-molecular weight
carbonyls, chlorinated compounds, and oxygenated solvents.
Aromatics are by far the most significant anthropogenic SOA precursors
(Grosjean and Seinfeld, 1989). Compounds like toluene, xylenes, trimethyl-benzenes,
etc., have been estimated to be responsible for 50-70% of the secondary organic aerosols
in urban areas. The experimental work of Odum et al. (1997) showed that the secondary
organic aerosol formation potential of gasoline could be accounted for solely in terms of
its aromatic fraction. The rest of the anthropogenic hydrocarbons (alkanes, paraffins, etc.)
have been estimated to contribute 5-20% to the SOA concentration depending on the
urban area.
Biogenic hydrocarbons emitted by trees are expected to be also an important
source of secondary organic particulate matter. Isoprene emitted mainly by deciduous
trees does not form organic aerosol under ambient conditions (Pandis et al., 1991) but
terpenes (- and -pinene, limonene, carene, etc.) and the sesquiterpenes are expected to
be major contributors to SOA in areas with significant vegetation cover. In the southeast
US -pinene and -pinene seem to dominate the monoterpene emissions, while in the
northern forests emissions are distributed more evenly among these two pinenes and 3carene, d-limonene, camphene, and myrcene (Geron et al., 2000). In some parts of
western forests, -pinene and 3-carene can be more abundant than -pinene.
The incremental aerosol reactivity (IAR) is one of available approaches to
quantify the ability of a given precursor to the ambient organic aerosol concentration. The
IAR is defined as the change in the secondary organic aerosol mass concentration per unit
of parent organic reacted. Griffin et al. (1999a) estimated the IARs of some typical SOA
precursors for typical atmospheric conditions (Table 1). The calculated reactivities varied
from a fraction of a g m-3 per ppb reacted for the aromatics to several g m-3 for
limonene and caryophyllene (a sesquiterpenes).
Table 1. Estimated Incremental Reactivities for Selected VOCs for Typical Atmospheric
Conditions (Griffin et al., 1999a)
Precursor
m-xylene
low-yield aromatics
high-yield aromatics
methylpropylbenzene
terpinolene
ocimene
linalool
-pinene
diethylbenzene
-carene
Sabinene
Terpinenes
-pinene
Limonene
Caryophyllene
Incremental Reactivity
(g m-3 ppb-1)
0.1-0.2
0.25
0.3-0.4
0.4-0.5
0.4-0.5
0.5-0.6
0.5-0.7
0.5-0.8
0.7-0.8
0.5-1.1
1-1.2
0.8-1.3
0.8-1.5
1.4-2.4
6-9
Our understanding of the ability of individual VOCs, at least at semi-quantitative level, to
serve as SOA precursors appears to be satisfactory.
2. Formation
There are two important steps in the formation of SOA. The first is the chemical
reactions in the gas phase leading to the production of the SOA compounds. These
reactions involve the parent VOC, its products, and O3, the OH radical, the NO3 radical,
NOx, etc. The second step is the reversible partitioning of the produced SOA compounds
between the gas and particulate phases. A number of processes are involved in this
second step including dissolution in the particulate aqueous or organic phase, adsorption,
condensation, etc. Small amounts of SOA may also nucleate forming new ultrafine
particles. Nucleation of SOA could be a potentially important process for the aerosol
number concentration, but is of negligible importance for the SOA mass concentration.
Both the chemical mechanisms leading to the formation of SOA compounds and
their partitioning have been the topic of numerous laboratory studies during the last
twenty years. The majority of the work has focused on the SOA production during the apinene reaction with ozone (there are roughly 20 published studies for this system). Other
systems investigated in some detail include the reactions of selected other biogenics with
ozone (see for example Yu et al., 1999, Koch et al., 2000, Hoffmann et al., 1998; Glasius
et al., 2000; Griffin et al., 1999), a-pinene photo-oxidation in the presence of NOx,
(Kamens and Jaoui, 2001), monoterpene reaction with the NO3 radical (Hallquist et al.,
1999), toluene photo-oxidation (Edney et al., 2000; Hurley et al., 2001), 1-tetradecene
and ozone reaction (Tobias and Ziemann, 2000; Tobias et al, 2000), methylene-cyclohexane and methyl-cyclo-hexene and ozone reaction(Koch et al., 2000), etc.
Development of detailed chemical mechanisms explaining the production of the
SOA compounds is an ongoing process (for a review see Calogirou et al., 1999). Because
of the chemical complexity, progress has been slow. A good example of this process is
the work of Jenkin et al. (2000) on the development of a “possible” mechanism for the
production of cis-pinic acid from the ozonolysis of  and -pinene. Additional work in
the development of these mechanisms is necessary as well as evaluation of the results
against multiple smog chamber experiments from different laboratories.
The partitioning of the SOA compounds between the gas and particulate phases is
a critical step in the overall SOA formation process. Most of the SOA compounds have
saturation vapor mixing ratios of the order of 1 ppb and are therefore semi-volatile.
Quantifying the fraction of these compounds that is in the particulate phase under given
conditions is a major challenge. The current approach suggested first by Pankow and coworkers and refined for SOA by Odum and co-workers assumes the formation of a
solution by the SOA compounds. The predicted SOA concentration in this framework is
sensitive to the vapor pressures of the SOA compounds, and their temperature
dependence, and to the activity coefficient of these compounds in solution. While these
physical properties can be calculated by group contribution methods (Jang et al., 1997) or
can be estimated by fitting the smog chamber results (Kamens et al., 1999), the
uncertainty of the estimated values can be as high as a factor of ten. The success of
efforts like the one by Kamens and Jaoui (2001) in reproducing not only the SOA mass
concentration time evolution but also of the major gas and aerosol products is
encouraging. Similar experiments and theoretical work will be necessary for the
reduction of the uncertainty of these models. Measurements of the physical properties of
the major SOA components will assist these efforts.
The anthropogenic and biogenic SOA compounds may interact. Increases in the
production of one of them may lead to increases in the concentrations of the other by
shifting their partitioning towards the particulate phase. Kanakidou et al. (2000)
suggested that the increase in anthropogenic SOA since pre-industrial times has resulted
in a threefold increase of the biogenic SOA concentrations globally.
Our understanding of the physicochemical processes leading to the SOA
formation has improved dramatically during the last decade. A number of models of
variable complexity are now available. Evaluation and continued development of these
models against multiple smog chamber data and ultimately field observations is the next
necessary step in this effort.
3. Composition
Numerous SOA compounds have been identified in laboratory studies (Table 2)
but only a few of them (mostly biogenics) have been identified in ambient air. The few
field studies that identified unique SOA tracer compounds were designed specifically to
achieve this objective and took place mostly in forests.
The list includes a number of unique compounds that could be used as tracers for
the quantification of the contribution of selected SOA precursors. Some of these
compounds (pinic acid, pinonic acid, norpinic acid, norpinonic acid, etc.) are the result of
Table 2. SOA Compounds Identified in Laboratory and Field Studies
Precursor
SOA Compound
Pinic acid
Norpinic acid
Hydroxypinonaldehydes
Pinonic acid
Norpinonic acid
Pinonaldehyde
Norpinonaldehyde
Hydroxy-pinonic acid
Pinic acid
-pinene
Norpinic acid
Pinonic acid
Norpinonic acid
Hydroxypina ketones
Hydroxy-norpinic acid
Hydroxy-pinonic acid
Pinic acid
sabinene
Sabinic acid
Norsabinic acid
Norsabinonic acid
Hydroxy-sabina-ketones
Sabine ketone
3
Pinic acid
 -carene
3-caric acid
hydroxy caronaldehydes
3-caronic acid
nor-3-caronic acid
caronaldehyde
hydroxy-3-caronic acid
Cyclohexene Adipaldehyde
6-oxohexanoic acid
adipic acid
glutaraldehyde
5-oxopentanoic acid
glutaric acid
succinic acid
Limonic acid
Limonene
Limononic acid
Hydroxy-limononic acids
1-tetradecene a-hydroxytridecyl
hydroperoxides
secondary ozonides
the oxidation of multiple precursors,
a-pinene
Ambient Air
Field Studies
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yu et al. (1999)
Kavouras et al. (1998;1999)
Yes
Yes
Yes
Yu et al. (1999)
Kavouras et al. (1998;1999)
Yes
Yu et al. (1999)
Kavouras et al. (1998;1999)
Yes
Yu et al. (1999)
Kavouras et al. (1998;1999)
Yes
Yes
Yes
but
others
(hydroxypinonaldehydes,
hydroxypinaketones, sabinic acid, etc.) are unique products of a given precursor.
The challenges in this effort are significant. First, there is the difficulty in
measuring the concentrations of these compounds. They not only are polar but their
concentrations appear to be only a few nanograms per cubic meter. Techniques that have
been developed in smog chamber studies can probably be extended to field studies. The
second difficulty is the partitioning of these compounds between the gas and particulate
phases. As most of them are semi-volatile their aerosol fingerprint changes continuously.
The third difficulty is the reactivity of these compounds. Some of the most volatile ones
(pinonaldehyde, nopinone, etc.) have been shown to have atmospheric lifetimes of the
order of a few hours. The reactivity of the less volatile ones has not been investigated in
any detail. Finally, there is an additional analytical pitfall. Tobias et al. (2000) discovered
that when they analyzed the aerosol produced from the reactions of 1-tetradecene and
ozone the hydroperoxides, peroxides, and secondary ozonides observed by thermal
desorption particle beam mass spectrometry (TDPBMS) thermally decomposed to more
volatile compounds including tridecanal, tridecanoic acid, and few unidentified products.
4. Environmental Conditions
The SOA concentrations are expected to be sensitive to the ambient temperature,
which will affect both the rates of the gas-phase reactions and the partitioning of the SOA
compounds. According to Kamens and Jaoui (2001), a 10oC change in the ambient
temperature can change the SOA concentration in the a-pinene/ozone system by as much
as a factor of two. Higher temperatures are expected to decrease the SOA yields in this
system.
Strader and Pandis (1999) suggested that for the ambient atmosphere the
increase in production rates with increasing temperature would partially offset the
evaporation of SOA. They suggested that intermediate temperatures (around 20oC in their
case) could be optimal for the SOA production. Bowman and collaborators proposed that
the change in partitioning of SOA at lower temperatures could lead to counterintuitive
behavior of the SOA concentration during the day, e.g., a maximum during the night even
without any nighttime production.
The role of relative humidity in the SOA production has yet to elucidated. Edney
et al. (2000) argued based on their laboratory measurements that the presence of aerosol
liquid water does not significantly increase or decrease SOA yields during the photo-
oxidation of toluene in the presence of NOx. The same lack of sensitivity to relative
humidity was reported for the a-pinene/ozone system by Kamens and Jaoui (2001). Jang
and Kamens (1998) estimated that the partitioning of a variety of semi-volatile organic
compounds (alkanes, alkanoic acids, PAHs, etc.) on secondary organic aerosol (formed
from the a-pinene reaction with ozone) was not sensitive to relative humidity. On the
other hand, water vapor was found to be an important reactant during the reactions of 1tetradecene and ozone (Tobias et al., 2000). Seinfeld et al. (2001) estimated that the SOA
concentration formed in the -pinene ozone system increases by roughly 25% as the
relative humidity increases from 0 to 90%. The corresponding changes for the other
biogenics were 40% for -pinene/ozone, and 10% of carene. A much larger change
(around a factor of 2) was predicted for the cyclohexene/ozone and sabinene/ozone SOA
because of the hydrophyllicity of their products.
SOA production usually requires sunlight. However, reaction between the nitrate
radical and -pinene, 3-carene, and sabinene leads to high conversion to aerosol,
indicating probable ambient aerosol formation at night when monoterpenes, continue to
be emitted and the nitrate radical concentration increases (Griffin et al., 1999b; Hallquist
et al., 1999). Even if the chemical production of SOA were slow during the night, the
SOA concentration could increase during the nighttime by condensation of the SOA
compounds that may exist in the gas phase.
High time resolution ambient measurements of OC and EC as well as of some of
the major SOA components will be necessary for the improvement of our understanding
of the role of environmental conditions in the SOA formation.
5. References
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F. Smith (2000) Impact of aerosol liquid water on secondary organic aerosol yields of
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Geron C., R. Rasmussen, R. A. Arnts, and A. Guenther (2000) A review and synthesis of
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